usb: chipidea: udc: .pullup is valid only when vbus is there

For chipidea, the IP must know vbus before the controller
begins to run. So the .pullup should only be called when
the vbus is there.

Tested-by: Marek Vasut <marex@denx.de>
Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>